This invention relates to an aircraft and more particularly to an aircraft which is capable of vertical take off and preferably also, vertical landing (VTOL).
Such aircraft are known which are of configurations which have come to be known as xe2x80x9ctailsittersxe2x80x9d, an example being disclosed in U.S. Pat. No. 5,289,994. In this previous specification, a VTOL aircraft is described which has a pair of fixed wings which provide lift in normal horizontal or cruise flight, and helicopter-like rotors or propellers to provide lift during vertical take off and landing, and thrust in cruise flight.
On the ground, the aircraft is adapted to sit on a tail structure thereof with a fuselage extending generally vertically.
Other examples of similar aircraft are disclosed in U.S. Pat. Nos. 2,622,826, 2,859,003, and U.S. Pat. No. 5,516,060. In the latter two prior proposals, jet thrust instead of rotor/propeller thrust is utilised. In each of these examples, the wings which provide lift during normal horizontal flight are fixed relative to a fuselage thereof, although the wings may be provided with ailerons and the like for cruise flight control.
Such fixed wings having a relatively high aspect ratio are capable of achieving good cruise efficiency especially in the generally low speed horizontal flight regime of such aircraft but the fixed spread position impairs hover and low speed stability and control in the vertical take off and landing mode.
According to one aspect of the invention we provide an aircraft having a fuselage and a pair of wings and being capable of vertical take off with the fuselage generally vertical and normal cruise flight with the fuselage generally horizontal, the wings being movable relative to the fuselage from a rearwardly swept position to which the wings are moved for vertical take off, to a spread position having a higher aspect ratio to which the wings are moved for normal cruise flight.
By virtue of the wings being so movable, improved low speed performance and control, particularly during and immediately after vertical take-off can be achieved with the wings in the rearwardly swept position.
It is well known in conventional aircraft which take off and land generally horizontally to provide so called swing wings, by means of which a wing spread position can be provided during take-off and landing, and a rearwardly swept position can be provided during high speed cruise flight. In an aircraft in accordance with the invention which is capable of at least vertical take off, an opposite regime is adopted.
By moving the wings during vertical take off, to a rearwardly swept position, the distance of wing control surfaces, such as ailerons/flaps or the like from a centre of gravity of the aircraft, is increased, to provide a lengthened moment arm to increase the control forces which can be achieved by moving the control surfaces.
Furthermore, with the wings in the rearwardly swept position, the space required for storage of the aircraft is reduced.
In general, VTOL aircraft of the kind with which the invention is primarily concerned are not required to achieve very high speed forward cruise flight, but may achieve cruise flight speeds in the range 150 to 200 knots. At such relatively low speeds the higher aspect ratio of the wings in the spread position provides an improved lift/drag ratio to optimise the efficiency of the aircraft during normal cruise flight.
If the invention was applied to an aircraft which was capable of higher speed forward flight, if desired in such high speed mode of operation, the wings may be swept rearwardly to an interim or fully swept position to reduce the aspect ratio and to improve efficiency at such higher forward speeds.
The aircraft of the invention may have an e.g. cruciform configuration tail structure including tail wings and, prior to take off, the aircraft may sit on the tail structure.
In a preferred arrangement, the aircraft is capable of both vertical takeoff and landing. Thus preferably the wings are moveable to the rearwardly swept position during landing as well as take-off.
Thrust at least for take-off, and landing where the aircraft is capable of vertical landing, may be provided by a rotor/propeller system, which may include a pair of contra-rotating rotors which mutually counteract torque reactions. Such rotor system may be provided at or adjacent a nose structure of the aircraft.
Alternatively or additionally, thrust at least for take-off, and landing where the aircraft is capable of vertical landing, may be provided by a jet thrust preferably provided in the fuselage e.g. at a tail end of the aircraft.
If desired, a rotor system may be used to achieve thrust during at least take-off, and landing where the aircraft is capable of vertical landing, and thrust may be provided by a jet thrust during normal cruise flight.
A rotor system is preferred for at least take-off, and landing where the aircraft is capable of vertical landing, for finer stability and control, for manoeuvring and hovering.
The aircraft may be used as an un-manned aerial vehicle.
According to a second aspect of the invention we provide a method of operating an aircraft having a fuselage and a pair of wings, and being capable of vertical take-off with the fuselage generally vertical and normal cruise flight with the fuselage generally horizontal, the method including moving the wings relative to the fuselage to a rearwardly swept position for vertical take-off, operating a thrust means to achieve vertical take-off, manoeuvring the aircraft to the normal cruise flight condition, and moving the wings relative to the fuselage to a spread position for normal cruise flight.
Preferably the wings are moved relative to the fuselage from the swept position to the spread position during the transition from vertical flight to normal cruise flight.
The method of the invention may include moving the wings from the spread position to the rearwardly swept position during transition from normal cruise flight to vertical flight prior to vertical landing.
The aircraft to which the method of the second aspect of the invention is applied, may have any of the features of the aircraft of the first aspect of the invention.